Intercommunication system



v INVENTOR Wmnnn RKDCH W. R. KOCH Filed April 29, 1948 INTERCOMMUNICATIQN SYSTEM March 28, 1950 ATTORNEY aten'teci Var. 28, i950 2,501,820 INTERCOMDIUNICTION SYSTEM Winield R. Koch, Marlton, N. J.; assignor to Radio Corporation of America, a corporation of Delaware Application April 29, 1948, Serial No. 24,071

20 Claims.

This invention relates to intercommunicating is the provision of novel intercommunicating systems of simple construction in which only a Single pair of conductors is used to carry the signals in either direction between a plurality of signal stations.

The above as well as other objects of the invention will be more readily understood from the following description of exemplifications thereof, reference being had to the accompanying drawings wherein:

Fig. 1 is a schematic showing of one form of interconnnunicating system embodying the invention;

Fig. 2 is a circuit diagram of one form of modulating arrangement for use with the invention; Fig. 3 is a circuit diagram of a different modiication of modulator arrangement for use with the invention;

Fig. 4 is a ycurve diagram illustrating a further type of modulation embodied in the invention;

Fig. 5 is a circuit diagram of one type of amplifier and demodulator elements according to the invention; and

Fig. 6 is a different modification of communication system incorporating the invention.

According to the invention, an intercommunieating system contains a plurality of stations, a single pair of conductors supplied with high frequency electric waves interconnecting all the stations, and arranged so that at any one of a plurality of stations the high frequency waves may be modulated to deliver to the conductors signalmodulated carrier currents which are converted into signal currents and fed to one or more other stations in which the signals are reproduced. By single pair of conductors is meant two different electrical conducting paths, one of which may merely be a ground connection `as in normally used to establish a complete electric circuit. With such a ground connection forming one of the conductors only one actual conductor need be used to interconnect the stations and to provide the pair of conductors.

. Referring to the drawing, the form of the invention there shown includes ,high frequency sources-I0, II having output leads I3, I4 for deliver-'ing high frequency e1ectrical. waves to the intercommunication conductors I6, I8, one of:

Whichmay be grounded as shown. One suitable tact 44.

form of the high frequency sources is the conventional vacuum tube oscillator. The output leads It, I4 may be connected across the line in parallel or in series as shown. A combined amplifier and demodulator indicated at 20 has input leads 22 and output leads 24 coupled to the line through capacitance 56 and choke 58 respectively for selectively amplifying and detecting signal-modulated waves appearing in the line and delivering back to the line amplified currents corresponding to the signals such as acoustic signals desired to be intercommunicated.

At desired portions of the line formed by the pair of conductors, a plurality of intercommunication Stations 30, 40, 50, G, 'l0 -are shown as connected. Stations 30, 4U, 60 are of one type and each includes a transducer 32, such as a dynamic or piezoelectric acoustic reproducer, a modulator 34 and a shifting switch 36. The switch 36 selectably connects the transducer 32 either to the line I6, I8 for producing acoustic signals corresponding to low frequency signal currents in the line or it connects the transducer 32 to the modulator 34 and the modulator to the line for causing signal modulation of the high frequency Waves supplied to the line by the' sources I0, Il.

The shifting switch 36 is shown as of single pole, double throw construction having a blade 38 movable from a normal position, shown at stations 30, 40 in which it is held against a contact 42 as by the bias spring 45, to an operated position shown at station 60, in which it may be held, manually for example, against a second con- The blade itself is connected to one lead of transducer 32 through choke coil 48, and the other lead of the transducer 32 is shown as connected to conductor I8. Capacitor 54 in connection52 serves to also connect the blade 38 with the other line conductor I6. Contact 42 is also connected to the conductor I6 and contact 44 is connected to ythe input ofmodulator 34 which may merely be a so-called crystal detector. The switch 3B functions as a push to I talk" switch.

The system of the invention may also include any number of one-Way talk or listen stations. In a talk station, such as '50, locally produced acoustic signals modulate the high frequency waves on the line for transfer to other stations only. In a listen station, such as 10, only listening to signals on the line can be effected.` Y In operation, one of the switches 36 may be "ac-` tuated tov the talk position as shown at station 6.9 and acoustic signals then supplied to the corresponding transducer. The high frequency currents supplied by sources ID and lI are thereby applied to the associated modulator 34 through capacitor 54, and at the same time, low frequency electric signals corresponding to the acoustic signals are also applied to the modulator. As a result, the electric waves beat together to form beat signals modulated in accordance with the acoustic signals supplied. This results from the well known heterodyne action of modulators such as crystal detectors, which are non-linear conductors. Under these conditions there accordingly appear in the line electric signals having beat frequencies corresponding to the sums ofthe fundamental and harmonic frequencies of the different waves supplied by sources III, II, as weill as beat frequencies corresponding to the differences between these fundamental and harmonic frequencies. The beat signals carry modulations or amplitude variations corresponding to the acoustic signals because of the simultaneous heterodyning with the electric signals supplied by the transducer.

All; the modulated beat signals are carried by the line and appear at the input 22 of the amplier and demodulator 2B which may be arranged to selectively amplify and demodulate a band of frequencies corresponding to one of the modulated beat signals. As a result, amplified electric signals corresponding to the original acoustic signals appear in the line and are delivered to all listening stations. Here they pass to the transducers where acoustic signals are reproduced for listeners to hear. Two way communication such as ordinary conversation may be had by holding switch 39 in its operated talk position when sending and releasing the switch to receive messages.

Although one form of modulator 34, as described above, is a crystal detector, the modulator may be of any other conventional construction such as the non-linear current conductors commercially lrnown as rectifiers or detectors. Examples of the so-called crystal type are those embodying a crystal of natural lead sulfide (galeria), germanium, selenium or other materials having a greater electrical conductivity in one direction than in the opposite direction. A1- ternative forms ofv modulator are those of the laminated type having a semi-conductive stratum speciall)7 formed and sandwiched between special conductive layers to provide similar non-linear conductivity as in the copper oxide, copper sultide or commercial selenium rectiiiers. Other examples of suitable modulators are space discharge devices such as the so-called vacuumtubes where currents pass in only one direction from an electron emitting cathode to an electron collecting plate. The vacuum" tubes may also include' one or more electron passage controlling grids which may be connected for effecting some or all of the heterodyning functions, as is Well known in the art. The space discharge modulators generally require a heat energy supply to actuate the cathode for emitting electrons.

Fig. 2 shows the details of one type of modulator arrangement utilizing a selenium rectifier. The modulator 34 is merely the rectifier 35 which has two terminal electrodes 31, 39 held against an intervening layer of selenium 4I on one face of which is formed the special so-called "barrier layer as is well-known in the art. One electrode 31 is directly connected to the switch terminal 44 and the other electrode, 39 connected to the.

line conductor I8 in the manner described in connection with Fig. 1. The arrangement of Fig. 2 is also suitable for modulators of the vacuum tube diode type where the diode connections are made as shown for the rectifier terminals 31, 39. The rectifier may be connected with its more highly conductive path, or polarity, in either direction.

Fig. 3 shows anotherform of modulating arrangement utilizing a grid control type of electron discharge tube. The electron discharge tube shown as a triode having an electron emissive cathode 3i, an electron collecting elec- 'trode or plate 82 and electron passage controlling grid. Switch 2--36 shown in the non-operated position in which movable contact blade 2-38 is held by bias 2-v45 against fixed contact where it connects the line conductor I6 with a transducer 32 through blocking choke 48 and connection 2|. The switch blade 2-38 may be moved to an operated position in which it engages fixed contact 274.4 and connects the line i6 to the plate 82 through blocking capacitor 55. The circuit to transducer 32 and tube 30 is completed by the conductor I9 and line conductor T8, as shown. Transducer connection 2i is also linked to tube grid 83 and may be connected to the cathode through grid return resistor 84 and cath-v ode return resistor 85. Plate voltage for operating the tube 80 may be supplied by having the high frequency Waves delivered by the respective sources to the line, of sufficiently high voltage, as for example about 10 volts R. M. S. Alternatively, a separate plate current source 86 may have its positive terminal connected between the blocking condenser 55 and the-plate 82 through av choke 49. The negative terminal of the plate current source may be grounded. Where no separate plate current source 8G is used, the choke 49 as well as the blocking capacitor may be omitted leaving the plate 82 directly connected to contact 2-44. If desired the separate plate current source may be included in a high frequency wave source I0, II or in the combined amplifier and demodulator 20. The transducer may then be isolated from the D. C. plate current by a blocking capacitor. The grid return resistor 84 establishes a return flow path for electrons emitted in the tube 80 and collected on the grid; this resistor is not needed where the grid return circuit established by the transducer 32 has a suciently low resistance. The cathode return resistor 85 develops a bias voltage when current passes through the tube and applies this voltage as a negative bias on the grid with respect to the cathode for limiting the current flow as desired. This resistor may also be eliminated if desired.

Cathode 8l generally requires heating for proper operation. This may be accomplished in the conventional manner, either indirectly or directly, b y a separate source of electric current. If desired the high frequency energy supplied to the line I6, I8 may be used for also heating the cathode,- particularly where cathode 8| is of the directly heated type requiring a very small amount of heating power. For such purpose a high frequency transformer may be connected to deliver high frequency currents from the line I6, I8 at a reduced voltage to suitable rectifier elements which in turn supply the cathode heating cui-rent.- Thecathode, even if of the directly heated type; may also be heated by unrectied low voltage line energy.

The switching circuit need not be in the plate circuit of Fig. 3, but may alternativelybe in the grid circuit and arranged to shift the connection of the transducer 32 to the line i6, I8, or to the grid 83. If desired a double-pole switch may be adapted to simultaneously switch both plate and grid circuits. The tube 80 may have additional grids such as screen grids, a suppressor grid and/or other signal mixing grids.

Also suitable for modulators are other types of non-linear conductors such as those that exhibit impedance changes in response to changes in applied voltage. Thus, tungsten conductors, or the silicon carbide elements described in U. S. Patent No. 1,822,742, granted September 8, 1931, may be used. With this type of non-linear conductor the electrical conductivity in opposite directions is substantially identical, and the curvilinear relationship between conductivity and voltage is in the form of two symmetrical oppositely curving sections on each side of the zero-voltage level. The A. C. signals to be modulated may be biased by a direct current signal large enough to carry all the signal variations without changing its direction to minimize any frequency doubling effect of oppositely curved non-linear conductivities on both sides of the zero signal level.

Fig. 4 is a curve diagram illustrating the modulating operation according to this form of the invention. Curve Sll represents a typical example of the current flowing through a non-linear resistor of the variator type, under Varying voltages. The curve has two symmetrical portions 9|, 92 joined at their zero-voltage points. Alternating current signals to be modulated if directly applied in unbiased condition to the Variator are represented by the simple Waves 93 and it will be noted that each cycle of the waves 93 brings the variator through first one and then the other of the oppositely curved conductivity zones 9|, 92. There accordingly tends to develop two separate modulation surges at each cycle of the signals to be modulated and the desired signals may be dimcult to derive from the modulated output without dividing the output frequencies in half. However, when the signals to be modulated are confined to only one curved portion of the conductivity cure such as 9|. or 92, as by superimposing on these signals a D. C. bias 94 sufficient to shift the A. C. signals to the condition shown at 95, for example, this difficulty is avoided. The D. C. bias may be supplied by any of the arrangements described above to `provide the D. C. plate current for the construction of Fig. 3.

The choke coils 48 are low pass blocking ele- 4ments used for increasing the impedance presented by the transducer 32 to the line where desirable to thereby diminish loading of the high frequency sources Ill, Il when the stations are in receiving or listening position as shown at 30, 40, 10, and the transducer is of low impedance to the high frequency waves. The blocking elements 48 function by acting as a high impedance to high frequency waves and at the same time constituting a low impedance to the signals supplied by the transducer. Other conventional low pass blocking elements such as low pass filters may be substituted or the blocking elements may be entirely omitted. Choke coil 58 is similarly used to diminish the loading effect of the output 240i amplifier-demodulator 2U on the high frequency supplies, where this feature is desired. The input 22 of the amplifier-demodulator may itself be of relatively high impedance as for example the normal input resistance of an electronic ampliei" circuit. so that it need not blocked.

Capacitors 54 are high pass blocking elements tc diminish the loading effect on a transducer supplying electric signals, of the connected circuit elements including the other transducers that are connected for receiving electric signals from the line. These blocking elements function by acting as a high impedance to the electric signals supplied by the local transducer 32 pre. venting them from directly reaching the line I6, i8, and at the Sametime acting as low impedance connections for the high frequency waves transmitted on the line to supply these waves to the modulator-lll.

Where the transducersare of the highimpedance type such-as the piezoelectric or polarized capacitor .types-and they do not appreciably load the high frequency supply, the low pass blocking elements 4 8 arenot needed. Station is shown as incorporating a high impedance transducer 33 in such a construction. The high impedance referred to is the impedance to the high frequency currents on the line and in those transducers having a high resistance, but also incorporating a capacitance of such magnitude as easily to pass the high frequencies, blocking elements may be retained. The blocking action may also be provided by forming a parallel resonant circuit with a capacitance such as the transducer capacitance, and tuning the circuit to approximately those frequencies to be blocked.

Where the line presents a relatively high im,v pedance to the low frequency electric signals supe plied by the transducers, and thereby does not tend .to shunt and decrease the effectiveness of the modulators 34, the blocking elements 54 are unnecessary. These blocking elements are also unnecessary in stations such as lll in which no talk function is provided.

Capacitors 56 are high pass blocking elements incorporated in the output connections of the supply sources i0, Il and in the input 22 of the amplier and demodulator 20. These elements diminish the loadingeffect of the connected' structures on the low frequencysignalssupplied by the amplifier and demodulator 2U. Where the input 22 of the amplifiera'nd demodulator isof the high impedance type such as a space discharge tube input, the blocking element 56 may be omitted, the selectivity of the amplifier-de modulator 20 being then relied on to prevent oscillation. I

Fig. 5 is a circuit diagram of one form of amplifier-demodulator 3--20 having a high impedance input 3-22. This arrangement includes a first high frequency amplifying stage including electron discharge tube 3-40, a selective coupling circuit including high frequency transformer 3-50 coupling the high frequency output of the rst stage to a demodulator stage includ'- ing detector tube 3--4|, and a final low frequency amplierstage including tube 3-42. The low frequency output is shown as supplied to nal output 3-24 through a low frequency coupling transformer 3-5I. The various stages are operated inconventional manner as shown. The input impedance of this arrangement is essentially that of the grid return resistor 3-,39 connected across the input 3-22 in series with by- Dass capacitor 3 46. Automatic volume control is shown las incorporated in the amplification by the connection 3-56gwhich supplies variable bias voltage to the input grid of tube 3f-4.0 from .the

7 detecten output 'th-rough 3"-'58-5 as is Well known.

Adequateii. selectivity. isf incorporated through thief-.highly selective*-band-lpasscharacteristics of high Yfrequency transfert-50; to preventuncontrolled". feedbackofY thery lowAV signal frequencies fronti output` 3-.24 tothe-input offstage 3--42 through thelineiandfinput 13-22. Uncontrolled feedback of. high frequency carrier from the output..'of:detector3'4l tto the input fsf-'22 is-pre` vented` bythe iii-tering 'of .the-detector `output -with condensers 31,59; v3'-6Il,l andiffdesired, achigh frequencyf bypass condenser 3`62 lin thel output of tube ils-42;

The amplifier-demodulator circuit 'z may be modiflediaslby usingianyotherztyp'e ofifdemodulaton:a1,cr.ysta1 detector. for example, or. byv inter changing-.the :number-of 'amplifica-tion. stages and the fsequenceof amplification and -demodulation Somero'r alli ofthe tubes maybe-combined 'as dif- -y efiiciency.. A coaxiarline in which .one of..the conductors is,substaritially completely surroundedgby-,the otheris lone-,example of the lowloss type. In those .forms where oneconductorof the line is grounded, only 44a single conductor suchas conductor I6 needbe speciallyYwiredvto` al1 .the sigatiolns.l an eifective ground .connection at each unit taking the place of conductor .|8., With the.

coaxialltype line, theouter conductor may be grounded andin any event neednot be, exteriorly insulated fromv ground if. the ,central conductor. isui'ijgro.unded.v

As examplesfor. the practical operation of. the

invention to communicate. signal variations hav.

ing frequencies in a rangebelow 3000.*cycles per` second', and without limitin'gthe invention .in any way, the sources I0, Il may sl'ppli?y currents having' frequencies of 30,000 and 50,000 cyclespei sec.- ond respectivelyand the amplifier. and demodulation 20 may be arrangedzfo'r selectively amplifyin'g' vand`demodul'a'tihgpa' 6000 cycle" band of frequencies, having` a mid-band frequency .of 80,000 cycles per second.' Other parameters may. also bel effectively used.A Thus, for example', the` sources.

I0; H' may supply energy jatv frequencies as hig'h as several' megacycles per second 'or' as lowl as about twice the highest signal frequency ,to be.

related derivative such as thesum. or .differencev between one frequency and two times the other frequency; When using the line1 tor transmit en ergyatonly relaiively low frequencies, of the.

order of 100,000'cycles-per-second` or lessythe reiiectin-gv and res'onating eifects 'ofthe transmission line can in'ge'neralv-be ignored`inasmuch-1as un-v less thelinebecomes about one-eighthofi a mile" long-1 or-- longer,I it A forms only -a'f'- negligible portion otfthcwaveleugthff The combined amplifler .anddemodulator may' beof anyconventionalv construction as, for example, the. ordinary intermediate frequency am'- plier, detector-'andwaudio amplifier stages of a. standard heterodynetype radio receiver. A radio of this type may be incorporated inthe system in this manner, so that the convertersection of 4the radio may be selectably connected for transferringxth'e incoming radio .signals :tothe listeningrstations The selected beat vfrequency of the systemmay be Varranged to be the same as kthe inter-mediate.; frequency of. the radio, normally centered: at'ab'out '455kilo'cycles per second.` An `automatic'A volume control arrangement may be included` inthe .'amplier, as vshown above for exampleg-.toi compensate for variations in intensity ofthe Vmodulated signals coming from stationsat differentidistances.from thev amplifier and de-A modulator/:ZUL The--amplier and demodulator mayalternatively ori concomitantly be located approximately centrally fbetween the ends-of .the line to reduce the .overall difference between the shortest. `and. longestsignal transfer paths and thereby :diminish signal intensity differences. A suitable form of xthe-invention includes the energy sources l0, H. and'theamplifier-demodulator 20 inI a single assembly conveniently located and having a simple two-conductor connection to-the line;

According to .amodication of the invention; the electrical energy supplied for signal modulationaiat the `intereommunicati'on stations 30,-A 40,1 50fand gfo'rsexample, -may'belra carrier current of 'only a single frequency. The operation of this form of the invention isf similar to that described above,- exceptl for :the fact that the amplifier-demodul'atori 20 :is arrangedl to" selectively receive anddemodulate arband-of frequencies centering about Vthe supply frequency. Inasmuch as thek supplyenergy `is -of relatively ylarge magnitude: with respect tothe,l modulated energy,the' -amplifier-fd'emodulatoi-rnay be arranged to apply all or most'` of its amplification to the demodulated sigh'- naltonmin'imize the possibility of overloading and distortionfin high frequency amplification stages.

In-fthissimplied .form .of the invention the' sig-r nal modulation may be provided by non-linear conductors or by: `other impedance' varying dc-A vicessuch as the.. conventional carbon ".micro'. phone. As: acoustic-signals are supplied to' such' a' microphone, the carbon particles are variabl'y compressed to Avarythe microphone impedance' andcorres'pondin'gly vary the amplitude of car' rier current passed. A separate acoustic reprodu'c'er may be used in such constructions.

FigqG shows one' embodiment of this phase of' the invention. A high frequency supp'ly 4--10 delivers highfrequency electric waves to a line con-v ductor 4`-I6 through blocking capacitor 56. Thesewaves Aare distributed' among a set of inter'- comrn'unication stationstwof of Vwhich are shown' atl-30, 4;-40", the circuit being completed by the; ground connectionsv shown. The stations include a variable impedance microphone 4-34, an' acoustic'signal reproducer 4--32, vand a switch' 36 biased-'to connect the' line with the reproducer 41-l32 and capable of being operated to discon-- nect the reproducer and substitute a connection to'Y the microphonev I4-34. Choke coil 48`-is insertedbetweenthe switch 36 vand the reproducerA 4-'321tovincrease its impedance to the high fre' quenciesin lined-' iB'where'desired;

An amplier-dem'odulator 4-20' has an input through a1-blocking "capacitor SF' and blocking' -chollze coil S respectively. The amplier-demodulator 4-20 may be similar to that shown in Fig. 5 above, and may have its input directly led into the detector stage through a band pass coupling transformer such as shown at 3-50. Alternatively the input may be through only a single parallel resonant circuit such as the output side of transformer' 3-55.

The operation of the form of the invention shown in Fig. 6 is similar to that described above in connection with Figs. 1 through 5. If desired the microphone -M and the reproducer 4-32 may be acoustically isolated from each other as in a conventional telephone hand-set for example, and the switch 36 omitted leaving them both connected on the line. In this arrangement the delivery of intercommunication signals to a microphone need not be preceded by a switch operation and a conversation may be carried on with greater ease.

The form of the invention shown in Fig. 1 may also utilize separate microphones and acoustic reproducers with suitable switching arrangements provided to properly shift from talk to listen In this type of construction the modulator may, in addition to the non-linear conductor types, also be of the variable impedance type described in connection with the carbon microphone.

Instead of the modulating systems described above, the intercommunicating stations may be provided with other-systems to produce frequency, phase or pulse modulation of the carriers in any conventional manner. Such modulating arrangements are well known to experts in the art and need no further description here.

While several exemplications of the invention have been indicated and described above, it will be apparent to those skilled in the art that other modications may be made without departing from the scope of the invention asset forth in fi the appended claims.

What is claimed is:

1. In an intercommunicationsystem for carrying signals in either direction between a plurality of signal stations: a pair of conductors for carrying the signals, said conductors forming 4a line connected to each station for delivering high frequency alternating electric waves to the stations; at least two stations having modulating means for connection to theline to produce signal-,modulated waves by modulating the delivered waves in accordance with low frequencyv signals supplied to the modulating means for intercommunication, and to supply said signal-modulated waves to the line; amplifying and demodulating structure having input and output connections to the line for receiving said signal-modulated electric waves from the lineal/1d delivering carrying the signals, said conductors forming a line connected to each station; supply structure connected to the line for delivering alternating electric waves of two different high frequencies to the line; at least two stations having modulating means including signal transducer elements for connection to the line to produce signal-modulated waves in which the modulations correspond to' low frequency signals supplied to the tran`s- 10 vducer elements for intercommunication; amplifying and demodulating structure having input and output connections to the line for receiving said signal-modulated electric waves and delivering back to the line amplified signal waves corretsponding to theflow frequency signal modulations; said last mentioned stations alsov including shift means connected for selectably connecting the transducing elements to the line .for receiving and reproducing signals carried assignal waves by the line, and selectably connecting the modulating means to the line for producing said signal-modulated waves. y

3. The combination as dened by claim 2 in which the amplifying and demodulating structure includes selector elements for limiting the amplification to waves in a band of frequencies corre.- sponding rto those resulting from the cross modulation between the two different high frequency waves delivered by the supply structure. t

4. The combination as defined by claim 2 in which the supply structure is connected to lone section of the line for delivering the high frequency waves to all the stations.

5. The combination as defined by claim 2y in which the line has a characteristic impedance and includes terminating structure having an impedance approximating the characteristic impedance of the line, to diminish resonance effects.

6. The combination as defined by claim 1min which the line has an effective electrical length corresponding to a small fraction of the wave length of the currents delivered by the supply structure to diminish the interference by reflections of these currents from an end of the line.

1 7.*The combination as defined by claim 1 in which the amplifying and demodulating struc- -ture input connection is to a single section of the line, andthe amplifying and demodulating struc'- ture output connection is to a single section. of the line, for providing the amplification and deimodulation'of the signal-modulated currents 'delivered by all the stations. v Y.

8. The combination as dened by claim-1 in which the modulating means is a Variable-load modulator. fr

9. The combination as dened by claiml in which the modulating means is a heterodyning modulator.

10. The combination as defined by claim .2 in which the transducer elements include an electro-acoustic apparatus for interchangeably con'- -verting acoustic energy into corresponding elec'- tric signals and electric signals into correspond'- ing acoustic energy. i:

1l. The combination as defined by claim 2 in which the amplifying and demodulating structure includes selector .elements for limiting the amplification to waves in a band of frequencies vcorresponding to those resulting from crossmodulation between the two different high frequency waves ,delivered by the supply structure; "the supply structure includes an output connection to one section of the line for delivering the high frequency waves to all the stations; the amplifying and demodulating structure input connection and output connection being each to-.single sections of the line for providing the amplifica'- tion and demodulation of the signal-modulated currents delivered by the stations; said output connection includes blocking structure for increasing the impedance presented by the output to high frequency currents in the line; the modplating means, is a rechner; the `transducer elements include low 'impedance electroacousticaprality of signal stations: a pair of conductors for carrying the signals, said conductors forming a line connected to each station for delivering high frequency alternating electric Waves to the stations; at least two stations having modulating means for connection to the line to produce signal-modulated waves in which the modulations correspond to low frequency signals supplied to the modulating means for intercommunication; amplifying and demodulating structure having input and output connections to the line for receiving said signal-modulated electric waves and delivering back to the line amplified currents corresponding to the low frequency signal modulations; said last mentioned stations also including receiving elements and selectably operable shift means connected for selectably connectlng the receiving elements to the line for receiving signals carried as signal currents by the line and selectably connecting the modulating means to the line for producing said signal-modulated waves.

13. In an intercommunication system for car-.- rying signals in either `direction between a plurality of signal stations: a pair of conductors for carrying the signals, said conductors forming a line connected to each station for delivering high frequency alternating electric -waves to the stations; at least two stations having modulating means for connection to the line to modulate said high frequency waves and produce signal-modulated Waves in which the modulations correspond to low frequency signals supplied to the modulating means for intercommunication; amplifying and demodulating structure having input and output connections to the line for receiving said signal-modulated electric waves and delivering back to the line amplied currents corresponding to the low frequency signal modulations; said last mentioned stations also including transducing elements and selectably operable shift means connected for selectably con, necting the transducing elements to :the line for receiving and vreproducing signals carried as signal currents by the line and selectably connect.- ing the modulating means to the line for producing said signal-modulated waves.

'14. In an intercommunication system for carrying signals in either direction between a plurality of signal stations: a pair of conductors for .carrying the signals, said conductors forming a line connected to each station; supply structure coupled to the line for delivering high frequency alternating electric Waves to the stations; at least two stations having modulating means for connection to the line to modulate said high frequency Waves and produce signal-modulated Waves in Which the modulations lcorrespond to low frequency signals supplied to the modulating means for intercommunication; amplifying and demodulating structure having input and output connections to the 'line for receiving said signal-modulated electric waves and delivering back to the line amplified currents corresponding to the low frequency signal modulations; said last mentioned stations also including transducing elements and selectably operable shift means connected for selectably connecting the transducing elements to the line for receiving and reproducing signals carried as signal currents by the line and selectably connecting the modulating means to the line for producing said signalmodulated waves.

15. The combination as dened by claim 14 in which the supply structure and the amplifying and demodulating structure form parts of a single structural combination having a single connection to each line conductor for supplying the high frequency Waves, the amplification and the demodulation for all the stations.

16. In a signal station construction for an intercommunication system having a conducting line connecting a plurality of signal stations: signal transmitting elements for supplying electrical signals to the line for transmission to a diierent station; said transmitting elements ineluding modulating means connected for supplying the electric signals by selectably modulating electric Waves carried by the line, in accordance with si-gnals delivered to the transmitting elements for intercommunication; and receiving elements connected for receiving demodulated electric signals carried by the same line and corresponding to the signals transmitted by a different station.

17. In a signal station construction for an intercommunication system having a conducting line connecting a plurality of signal stations: signal transmitting elements for supplying electrical signals to the line for transmission to a different station; said transmitting elements including modulating means for supplying the elec.- tric signals by modulating electric waves carried by the line to the transmitting elements in accordance with signals to be transmitted, for operation essentially by the electric wave energy supplied from the line; receiving elements for receiving demodulated electric signals carried by the line and corresponding to the signals transmitted Iby a different station; and selectably operable shift means for selectably connecting the modulating means to the line for transmitting signals, and selectably connecting the receiving elements to the line for receiving signals.

18. The combination as defined by claim 17 in which the signal transmitting elements include electro-acoustic transducing means for delivering electric signals to the modulating means corresponding to supplied acoustic signals, and delivering acoustic signals corresponding to supplied electric signals; said transducing means being connected to the shift means to selectably form part of the transmitting elements for trans'- mitting signals and part of the receiving elements for receiving signals.

19. In a method of intercommunicating signals from a rst signal station to at least one other signal station over a single pair of conductors interconnecting the stations: supplying high frequency electric Waves to the pair of conductors for transmission to the stations; modulating the high frequency waves at said first signal station in accordance with low frequency signals to be communicated to form signal modu- Ilated electric waves carried by the conductors; amplifying and demodulating the signal modulated electric waves carried by the conductors; land supplying the demodulated amplied output to the conductors for communication to and receptionby at least one other station.

20. In an intercommunication system for carrying signals in either direction between a plurality of signal stations: a pair of conductors for carrying the signals, said conductors forming a line connected to each station for delivering high frequency alternating electric waves to the stations, receiving modulations of said Waves from at least one of the stations, and also carrying 10W frequency alternating electric Waves; supply elements connected for supplying said high frequency Waves to the line; at least one station having a modulator connected for selectably modulating the Waves delivered by the line in accordance with 10W frequency signals supplied to the modulator for intercommunication; amplifying and demodulating structure having input and output connections to the line for receiving said signal-modulated electric waves from the line and delivering back to the line amplified signal Cil Waves corresponding to the 10W frequency signal modulations; said last mentioned station also including transducer elements connected for receiving said signal Waves and reproducing said 10W frequency signals.

WINFIELD R. KOCH.

REFERENCES CTED The following references are of record in the 19 le of this partent:

UNITED STATES PATENTS Number Name Date 1,361,488 Osborne Dec. '7, 1920 1,579,256 Smythe Apr. 6, 1926 1,633,100 Hersing June 21, 1927 1,666,738 Hartley Apr. 17, 1928 1,746,808 Wolfe Feb. 11, 1930 1,885,010 Day Oct. 25, 1932 

